This invention is generally directed to processes for charging imaging members, such as photoreceptors, photoconductive imaging members and dielectric charge receivers, for ionography. More specifically, in embodiments the present invention relates to processes for charging photoconductive imaging members, especially and preferably layered imaging members by ionic conduction and wherein, for example, corona charging and discharging devices together with their known disadvantages can be avoided and/or minimized. Embodiments of the present invention include a process for the ion transfer charging of photoconductive imaging members, which process comprises contacting a liquid developer, or liquid developer fluid with the surface of the imaging member; and applying a voltage while moving, such as rotating the imaging member, thereby enabling the transfer of ions, preferably of a single sign, such as positive or negative, from the liquid/imaging member interface to the imaging member. The developer selected in embodiments contains mixed surfactants that, for example, increase the conductivity of the liquid developer by orders of magnitude relative to unmixed surfactant developer containing solutions, and wherein there is promoted ion transfer charging at high process speeds and with lower surfactant loading. Examples of developers selected include those containing mixed surfactants or charge directors of polymeric ammonium HBr salts, preferably poly[H,N-dimethyl-N-ethyl methacrylate ammonium bromide (A block)-co-2-ethylhexyl methacrylate (B block)] and salicylic aluminate, and more specifically, hydroxybis[3,5-di-t-butyl salicylic aluminate monohydrate] (ALOHAS), include those as illustrated in U.S. Pat. No. 5,366,840 and U.S. Ser. No. 065,414, now U.S. Statutory Invention Registration H1483, the disclosures of which are totally incorporated herein by reference. For example, a liquid developer with an 80/20 mixture of poly[N,N-dimethyl-N-ethylmethacrylate ammonium bromide (A block)-co-2-ethylhexyl methacrylate (B block)] and ALOHAS enables a charging fluid conductivity of about 100 times greater than the same concentration of the individual components. More specifically, charge directors include AB diblock, ABA triblock, BAB triblock copolymers or mixtures thereof with a M.sub.w of from, for example, about 2,000 to about 250,000, and wherein, for example, the A blocks are comprised of the repeat units of N,N-dimethyl-ammonium-N-ethyl methacrylate bromide sale, and the B block is comprised of repeat units of 2-ethylhexyl methacrylate, reference the block copolymer poly[N,N-dimethyl-N-ethylmethacrylate ammonium bromide (A Block)-co-2-ethylhexyl methacrylate (B Block)]. The diblock ammonium bromide copolymers are illustrated in U.S. Ser. No. 065,414, now U.S. Statutory Invention Registration H1483; the ABA triblocks in copending application U.S. Ser. No. 231,086 now U.S. Pat. No. 5,484,679; and the BAB triblocks in copending application U.S. Ser. No. 519,265, pending the disclosures of which are totally incorporated herein by reference. Also, in embodiments the liquid charging fluid is essentially free of thermoplastic resin and pigment.
The charging of photoconductive imaging members by means of corona discharge methods is known, however, a number of disadvantages are associated with these methods, such as the generation of ozone, the use of high voltages, such as from about 6,000 to about 7,000 volts, which requires the use of special insulation, maintenance of the corotron wires at added costs, low charging efficiency, the need for erase lamps and lamp shields, and the like. Since it can be a health hazard in some instances, ozone is removed by passage through a filter. Corona charging generates oxides of nitrogen which desorb eventually from the corotron surfaces and eventually oxidize the transport molecule thereby adversely effecting the electrical properties of the photoreceptor. These can show up as print deletions.
Generally, the process of electrostatographic copying is initiated by placing a substantially uniform electrostatic charge on a photoreceptive member. Subsequent to this charging, imaging is accomplished by exposing a light image of an original document onto the substantially uniformly charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges the photoconductive surface thereon in areas corresponding to nonimage areas in the original document while maintaining the charge in image areas, thereby creating an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by depositing charged developing material onto the photoreceptive member such that the developing material is attracted to the charged image areas on the photoconductive surface. Thereafter, the developing material is transferred from the photoreceptive member to a copy sheet or to some other image support substrate for creating a visible image, which may be permanently affixed to the image support substrate, thereby providing a reproduction of the original document. In a final step in the process, the photoconductive surface of the photoreceptive member can be cleaned to remove any residual developing material which may be remaining on the surface thereof in preparation for successive imaging cycles.
Liquid developers are generally known including developers with the charge directors mentioned herein.
The electrostatographic copying process described herein is well known and is commonly used for light lens copying of an original document. Analogous processes also exist in other electrostatographic printing applications such as, for example, digital laser printing where a latent image is formed on the photoconductive surface via a modulated laser beam, or ionographic printing, and reproduction where charge is deposited on a charge retentive surface in response to electronically generated or stored images.
In addition to charging the imaging surface of an electrostatographic system prior to exposure, corona devices are used to perform a variety of other functions in the electrostatographic process. For example, corona generating devices aid in the transfer of an electrostatic toner image from a reusable photoconductive imaging member to a transfer member such as paper; the tacking and detacking of the transfer member to and from the imaging member; and the conditioning of the surface of the imaging member prior to, during, and after deposition of toner thereon to improve the quality of the electrostatographic copy produced thereby. Each of these functions can be accomplished by a separate and independent corona generating device. The relatively large number of devices within a single machine necessitates the economical use of corona generating devices.
Various types of charging devices have been used to charge or precharge the surface of a photoconductive member. Corona generating devices are used extensively, wherein a voltage of 2,000 to 10,000 volts may be applied across an electrode to produce a corona spray which imparts electrostatic charge to a surface situated in close proximity thereto. One particular corona generating device includes a single corona generating electrode strung between insulating end blocks mounted on either end of a channel formed by a U-shaped shield or a pair of spaced side shield members. The corona generating electrode is typically a highly conductive, elongated wire positioned opposite the surface to be charged. In other conventional corona generating devices, the corona generating electrode may also be in the form of a pin array. Another device, frequently selected to provide for a more uniform charging and to prevent overcharging includes two or more corona generating electrodes with a control grid comprising a screen having a plurality of parallel wires or a plate having multiple apertures positioned between the corona generating electrodes and the photoconductive member. In this device, a potential having the same polarity as that applied to the corona electrodes but having a much smaller voltage magnitude, usually about a few hundred volts, is applied to the control grid to suppress the electric field between the control grid and the corona electrodes, markedly reducing the ion current flow to the photoconductive member.
Yet another type of corona generating device is described in U.S. Pat. No. 4,086,650 wherein a corona discharge electrode is coated with a relatively thick dielectric material, such as glass, for substantially preventing the flow of conduction current therethrough. In this device, the delivery of charge to the photoconductive member is accomplished by a displacement current or by capacitive coupling through the dielectric material. The flow of ions to the surface to be charged is regulated by means of a DC bias applied to the shield of the corona generating device. In operation, an AC potential of approximately 5,000 to 7,000 volts is applied to the coated electrode at a frequency of about 4 KHz to produce an actual corona generating current of approximately 1 to 2 milliamperes. This device has the advantage of providing a uniform charge to the photoconductive member using a charge generating device that is highly insensitive to contamination by dirt and, therefore, does not require repetitive cleaning or other maintenance requirements.
One problem associated with corona generating devices occurs in the presence of the generated corona, wherein a region of high chemical reactivity is also produced such that new chemical compounds may be synthesized. This chemical reactivity correspondingly causes a build up of chemical growth on the corona generating electrode as well as other surfaces adjacent thereto. After a prolonged period of operation, these chemical growths may degrade the performance of the corona generating device and also the entire electrostatographic machine.
Free oxygen, ozone, and other corona effluents, such as nitrogen oxide and nitrogen oxide species, can be produced in the corona region. These nitrogen oxide species react with solid surfaces. In particular, it has been observed that, in some instances, these nitrogen oxide species are adsorbed by the conductive control grid, the shield, shield members and other components of the corona generating device. The adsorption of nitrogen oxide species occurs even though the corona generating device may be provided with a directed air flow during operation for removing the nitrogen oxide species as well as controlling ozone emissions. During the process of collecting ozone, directed air flow may exacerbate problems by carrying the nitrogen oxide species to an affected area of the corona generating device or even to some other machine part.
The reaction of corona generating process byproducts, such as nitrogen oxide, with the shield, the control grid, or other corona generating device components can result in corrosive buildup and deposition on the surfaces thereof. These deposits can cause problems, such as nonuniform photoreceptor charging, manifested by side-to-side density variations, or dark and light streaks in an output copy. Also, depending on environmental conditions, deposits may charge up and effectively increase the shield or screen voltage resulting in similar nonuniformity defects. Extreme cases of corrosion can lead to arcing between the corona generating electrode and the screen on the shield members.
Another problem associated with corona generating devices operating in an electrostatographic environment results from toner accumulation on the surface of the corona generating electrode as well as surfaces adjacent thereto. The spots of accumulated toner, being a dielectric in nature, tend to cause localized charge buildup on the interior surfaces of the shield which produces current nonuniformity and reduction in corona current. Localized toner accumulations on the insulating end blocks which support the wire electrode also cause sparking.
Moreover, adsorption can be a physically reversible process such that the adsorbed nitrogen oxide species are gradually desorbed when a machine is turned off for an extended period of idleness. The adsorbed and desorbed species are both nitrogenous but not necessarily the same, that is there may be a conversion of NO.sub.2 to HNO.sub.3. When the operation of the machine is resumed, a copy quality defect, commonly referred to as a parking deletion, can result wherein a line image deletion or a lower density image is formed across the width of the photoreceptor at that portion of its surface resting opposite the corona generating device during the period of idleness. It is believed that the nitrogen oxide species interact with the surface of the photoreceptor to increase the lateral conductivity thereof such that the photoreceptor cannot effectively retain a charge in image configuration. This phenomenon basically causes narrow line images to blur or to wash out so as to not be developed as a toner image.
In corona generating devices, it has been found that the material from which the components, such as the shield or control grid, are fabricated has an effect on the severity of parking deletions. In the prior art, stainless steel materials have commonly been used as shields. Other materials, such as corrosion resistant ferrous materials which prevent the rapid oxidation of the component material and the concurrent loss of performance of the corona generator, have met with limited success, primarily due to the corrosive effect of the corona produced by the device.
In other attempts to reduce the problems associated with corona charging, considerable effort has been accomplished to reduce the adsorption of nitrogen oxides species by device components via the application of electrodag coatings to the surfaces thereof. These coatings typically include a reactive metal base such as nickel, lead, copper, zinc or mixtures thereof. These reactive metal base materials tend to absorb, or form harmless compounds with the nitrogen oxide species. However, parking deletion problems have continued, in some instances, due, for example, to the failure of the electrodag materials to continue to absorb or form harmless compounds with the nitrogen oxide species over time. In addition, certain components needed to address this problem are costly to fabricate.
Thus, the problem of chemical growth buildup in and around corona generating devices has been addressed by providing coating materials that are less prone to chemical attack. While adequately addressing the problem, such materials have substantially increased the cost of corona generating devices. Various forms of corona generating devices have been described for use in electrostatographic reproduction machines.
U.S. Pat. No. 4,258,258 discloses a corona generating device having a corona generating electrode supported between a pair of end block assemblies. Each end block assembly defines a space for the passage of the electrode, and nonconductive inserts for surrounding the electrodes that are seated in the spaces of the end block assemblies. The nonconductive inserts are made from a high dielectric strength material that is also resistant to a corrosive atmosphere. The inserts are easily and inexpensively replaced so as to protect the end block assemblies from the effects of high voltage applied to the corona electrode. Also mentioned with respect to corona generating devices are U.S. Pat. Nos. 4,585,320; 4,585,322 and 4,585,323. Also of interest are U.S. Pat. No. 2,987,660, see for example column 2, lines 50 to 68, column 3, lines 49 to 70, and specifically column 3, lines 59 to 61, wherein water is mentioned as a conductive liquid; U.S. Pat. Nos. 3,394,002; and 2,904,431.
Generally, layered photoresponsive imaging members are described in a number of U.S. patents, such as U.S. Pat. No. 4,265,900, the disclosure of which is totally incorporated herein by reference, wherein there is illustrated an imaging member comprised of a photogenerating layer, and an aryl amine hole transport layer. Examples of photogenerating layer components include trigonal selenium, metal phthalocyanines, vanadyl phthalocyanines, and metal free phthalocyanines. Additionally, there is described in U.S. Pat. No. 3,121,006 a composite xerographic photoconductive member comprised of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. The binder materials disclosed in the '006 patent comprise a material which is incapable of transporting for any significant distance injected charge carriers generated by the photoconductive particles.
Moreover, there are disclosed in U.S. Pat. No. 4,419,427 electrographic recording mediums with a photosemiconductive double layer comprised of a first layer containing charge carrier perylene diimide dyes, and a second layer with one or more compounds which are charge transporting materials when exposed to light, reference the disclosure in column 2, beginning at line 20.
Illustrated in copending patent application U.S. Ser. No. 332,313, pending the disclosure of which is totally incorporated herein by reference, is a process for charging layered imaging members by the transfer of ions thereto from an ionically conductive medium, wherein the ionically conductive medium is comprised of a liquid or a gel such as a hydrogel of a polyacrylate, polyacrylamide, polyvinylether, polypyrrolidinone, or polyhydroxyethylmethacrylate.
Illustrated in now U.S. Pat. No. 5,485,253, the disclosure of which is totally incorporated herein by reference, is a corona generating device and, more particularly, a reusable corona charging apparatus for use in an electrostatographic printing machine to generate a flow of ions onto an adjacent imaging surface so as to alter the electrostatic charge thereon.
In U.S. Ser. No. 065,414, now U.S. Statutory Invention Registration H1483; U.S. Pat. No. 5,306,591 and U.S. Pat. No. 5,308,731, the disclosures of which are totally incorporated herein by reference, there is illustrated a liquid developer comprised of a nonpolar liquid, thermoplastic resin particles, a nonpolar liquid soluble ionic or zwitterionic charge director, and a charge adjuvant comprised of an aluminum hydroxycarboxylic acid, or mixtures thereof.
The disclosures of each of the above patents and patent applications are totally incorporated herein by reference.
Illustrated in now U.S. Pat. No. 5,510,879, the disclosure of which is totally incorporated herein by reference, is a process for charging layered imaging members by the transfer of ions thereto from an ionically conductive medium.